Method and apparatus for demodulating signals processed in a transmit diversity mode

ABSTRACT

Demodulator architectures for processing a received signal in a wireless communications system. The demodulator includes a number of correlators coupled to a combiner. Each correlator typically receives and despreads input samples (which are generated from the received signal) with a respective despreading sequence to provide despread samples. Each correlator then decovers the despread samples to provide decovered “half-symbols” and further demodulates the decovered half-symbols with pilot estimates to generate correlated symbols. The decovering is performed with a Walsh symbol having a length (T) that is half the length (2T) of a Walsh symbol used to cover the data symbols in the transmitted signal. The combiner selectively combines correlated symbols from the assigned correlators to provide demodulated symbols. One or more correlators can be assigned to process one or more instances of each transmitted signal. The pilot estimates used within each assigned correlator to demodulate the decovered half-symbols are generated based on the signal instance being processed by that correlator.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a Continuation Applicationclaiming priority to patent application Ser. No. 09/594,466 entitled“Method and Apparatus for Demodulating Signals Processed in a TransmitDiversity Mode” filed Jun. 14, 2000 now U.S. Pat No. 6,628,702, having acommon assignee with the present application and hereby expresslyincorporated by reference herein.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to data communications. More particularly,the present invention relates to method and apparatus for efficientlydemodulating signals that have been processed and transmitted in adiversity mode.

II. Description of the Related Art

In a typical digital communications system, data is processed,modulated, and conditioned at a transmitter unit to generate a modulatedsignal that is then transmitted to one or more receiver units. The dataprocessing may include, for example, formatting the data into aparticular frame format, encoding the formatted data to provide errordetection and/or correction at the receiver unit, channelizing (i.e.,covering) the coded data, and spreading the channelized data over thesystem bandwidth. The data processing is typically defined by the systemor standard being implemented.

At the receiver unit, the transmitted signal is received, conditioned,demodulated, and digitally processed to recover the transmitted data.The processing at the receiver unit is complementary to that performedat the transmitter unit and may include, for example, despreading thereceived samples, decovering the despread samples to generate decoveredsymbols, and decoding the decovered symbols.

In some communications systems, data is processed and redundantlytransmitted over two (or possibly more) antennas to provide transmitdiversity. The processing may include, for example, covering the datafor each antenna with a particular channelization code (e.g., aparticular Walsh symbol). In some systems, the data for one or moreantennas may also be reordered prior to the channelization. Due tomultipath and other phenomena, the transmitted signals may experiencedifferent path conditions and may arrive at the receiver unit atdifferent times. If the transmit antennas are spaced sufficiently farapart, then the received signals from the antennas tend to fadeindependently. Each transmitted signal may also reach the receiver unitvia multiple signal paths. The receiver unit is then required toreceive, track, and process one or more instances of each transmittedsignal, and to combine the results from the processed signal instancesto recover the transmitted data. On the downlink, the processingtypically includes tracking a pilot that has been transmitted along withthe data, and using the recovered pilot to demodulate data samples.

The signal processing (e.g., demodulation) to process multipletransmitted signals, and multiple instances of such signals, can becomplicated. Moreover, transmit diversity is typically provided on thedownlink, and user terminals are required to support such a mode. Theuser terminals are typically more impacted by complexity and costsconsiderations. Therefore, techniques that can be used to efficientlydemodulate signals that have been processed and transmitted in adiversity mode are highly desirable.

SUMMARY OF THE INVENTION

The present invention provides demodulator architectures, demodulators,and receiver units for processing signals that have been processed andtransmitted in a transmit diversity mode. When operating in the transmitdiversity mode, data symbols are typically covered with a channelizationcode (e.g., a Walsh symbol) having a length (2T) that is twice thelength (T) of the channelization code used to cover the data symbols inthe non-transmit diversity mode. The demodulator architectures of theinvention exploit this property and perform partial processing (e.g.,despreading, decovering, pilot demodulation, or a combination thereof)on each fraction of a channelization symbol period of 2T. The processed“partial-symbols” are then appropriately combined to generate thedemodulated symbols. By performing partial processing on each fraction(e.g., each half) of the symbol period of 2T, computational complexityand costs can be reduced and performance may be improved. For example,with the present invention, the pilot demodulation in each assignedcorrelator (i.e., finger) can be performed based only on pilot estimatesgenerated by that correlator, whereas conventional techniques mayrequire pilots from multiple correlators. Other advantages are describedbelow.

An embodiment of the invention provides a demodulator for processing areceived signal in a wireless communications system. The demodulatorincludes a number of correlators coupled to a combiner. Each correlatortypically receives and despreads input samples with a respectivedespreading sequence to provide despread samples. The input samples aregenerated from the received signal. Each correlator then decovers thedespread samples to provide decovered “partial-symbols” and furtherdemodulates the decovered partial-symbols with pilot estimates togenerate correlated symbols. The decovering is performed with achannelization symbol (e.g., a Walsh symbol) having a length (e.g., T)that is a fraction (e.g., half) the length 2T of the channelizationsymbol used to cover the data symbols in the received signal. Thecombiner receives and selectively combines correlated symbols from theassigned correlators to provide demodulated symbols.

In the transmit diversity mode of a CDMA-2000 or W-CDMA standard (whichare identified below), the received signal includes a pair of signalstransmitted from a pair of antennas. One or more correlators can then beassigned to process at one or more instances of each transmitted signal.Each assigned correlator processes the received signal to recover pilotestimates corresponding to the signal instance being processed. Thepilot estimates are then used within the assigned correlator todemodulate the decovered partial-symbols.

A specific embodiment of the invention provides a demodulator thatincludes a number of correlators coupled to a combiner. Each correlatortypically includes a despreader, a decover element, a complexmultiplier, and a switch coupled in series. The despreader receives anddespreads input samples with a particular despreading sequence toprovide despread samples, and the decover element decovers the despreadsamples to provide pairs of decovered half-symbols. The decovering isperformed with a Walsh symbol W having a length (T) that is half thelength (2T) of a Walsh symbol W_(STS) used to cover the data in thereceived signal. (Space-Time Spreading (STS) is a transmit diversitymode defined by the CDMA-2000 standard.) One pair of decoveredhalf-symbols is provided for each Walsh symbol period of 2T. The complexmultiplier then demodulates the decovered half-symbols with a pilotrecovered by the correlator to provide demodulated half-symbols.

The switch provides a first combination of decovered half-symbols foreach Walsh symbol period of 2T in a first (e.g., even) symbol stream anda second combination of decovered half-symbols for each Walsh symbolperiod of 2T in a second (e.g., odd) symbol stream. The combinercombines the first symbol streams from the correlators to provide afirst (even) output symbol stream, and further combines the secondsymbol streams from the correlators to provide a second (odd) outputsymbol stream.

In one design of this specific embodiment, the multiplier in eachcorrelator performs a dot product and a cross product between thedecovered half-symbols and the pilot to provide “dot” symbols and“cross” symbols, respectively. The combiner can then be designed toselectively combine the dot and cross symbols for each Walsh symbolperiod of 2T to provide the demodulated symbols for the first and secondoutput symbol streams.

Another specific embodiment of the invention provides a demodulator thatalso includes a number of correlators coupled to a combiner. Eachcorrelator typically includes a despreader, a decover element, first andsecond summers, and first and second complex multipliers. The despreaderreceives and despreads input samples with a particular despreadingsequence to provide despread samples, and the decover element decoversthe despread samples to provide pairs of decovered half-symbols. Again,the decovering is performed with a Walsh symbol W having a length (T)that is half the length (2T) of a Walsh symbol W_(STS) used to coverdata symbols in the received signal, and one pair of decoveredhalf-symbols is generated for each Walsh symbol period of 2T.

Each correlator typically further includes a switch coupled to thedecover element. The switch provides decovered half-symbolscorresponding to the first half of the Walsh symbol period of 2T to afirst output and decovered half-symbols corresponding to the second halfof the Walsh symbol period of 2T to a second output. Each summer thenoperatively couples to the outputs of the switch and combines each pairof decovered half-symbols in a particular manner to provide a decoveredsymbol. Each multiplier then demodulates the decovered symbols from arespective summer with a respective pilot to provide a respective symbolstream.

The combiner receives the first and second symbol streams from the firstand second multipliers, respectively, of each assigned correlator,combines the first symbol streams from all assigned correlators toprovide a first output symbol stream, and further combines the secondsymbol streams from all assigned correlators to provide a second outputsymbol stream.

Another embodiment of the invention provides a method for processing areceived signal in a wireless communications system. The received signalcan include a pair of signals transmitted from a pair of antennas. Inaccordance with the method, input samples are generated from thereceived signal. At least one signal instance of each transmitted signalis then processed to provide correlated symbols. The processing for eachsignal instance typically includes despreading the input samples with aparticular despreading sequence associated with the signal instancebeing processed to provide despread samples, decovering the despreadsamples to generate decovered partial-symbols (e.g., half-symbols), anddemodulating the decovered partial-symbols with pilot estimates togenerate the correlated symbols for the signal instance. Again, thedecovering is performed with a Walsh symbol W having a length (e.g., T)that is a fraction of (e.g., half) the length (2T) of a Walsh symbolW_(STS) used to cover the data in the received signal. The correlatedsymbols for all signal instances being processed are then selectivelycombined to provide demodulated symbols.

The invention further provides other demodulator architectures,correlators, demodulators, receiver units, and methods to processsignals that have been processed and transmitted in a transmit diversitymode

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a simplified block diagram of a communications system in whichthe present invention may be implemented;

FIG. 2 is a block diagram of a modulator that can be used to process adownlink data transmission in a transmit diversity mode in accordancewith CDMA-2000 standard;

FIG. 3 is a diagram of a complex multiplier;

FIG. 4 is a block diagram of a conventional demodulator architecturethat can be used to demodulate a downlink data transmission that hasbeen processed in the transmit diversity mode; and

FIGS. 5, 6, and 7 are block diagrams of three specific embodiments of ademodulator architecture of the invention, which are also capable ofdemodulating the downlink data transmission that has been processed inthe transmit diversity mode.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

FIG. 1 is a simplified block diagram of an embodiment of acommunications system 100 in which the present invention may beimplemented. At a transmitter unit 110, traffic data is sent, typicallyin frames or packets, from a data source 112 to a transmit (TX) dataprocessor 114 that formats, encodes, and processes the data. TX dataprocessor 114 typically further processes signaling and pilot data,which is then combined (e.g., added, or time division multiplexed) withthe processed traffic data to generate composite data. A modulator (MOD)116 then receives, channelizes (i.e., covers), and spreads the compositedata to generate symbols that are then converted to analog signals. Theanalog signals are filtered, (quadrature) modulated, amplified, andupconverted by a transmitter (TMTR) 118 to generate one or moremodulated signals, which are then transmitted via respective antennas120 to one or more receiver units.

At a receiver unit 130, the transmitted signals are received by anantenna 132 and provided to a receiver (RCVR) 134. Within receiver 134,the received signal is amplified, filtered, downconverted, quadraturedemodulated, and digitized to provide inphase (I) and quadrature (Q)samples. A demodulator (DEMOD) 136 then receives, despreads, anddecovers the samples to generate decovered symbols. In certain designs,demodulator 136 further demodulates the decovered symbols with pilotestimates to generate demodulated symbols. The demodulated symbols arethen decoded and processed by a receive (RX) data processor 138 torecover the transmitted data. The despreading, decovering, decoding, andprocessing at receiver unit 130 are performed complementary to thespreading, covering, coding, and processing at transmitter unit 110. Therecovered data is then provided to a data sink 140.

The signal processing described above supports transmissions of voice,video, packet data, messaging, and other types of communication in onedirection. A bi-directional communications system supports two-way datatransmission. However, the signal processing for the other direction isnot shown in FIG. 1 for simplicity.

Communications system 100 can be a code division multiple access (CDMA)system, a time division multiple access (TDMA) communications system(e.g., a GSM system), a frequency division multiple access (FDMA)communications system, or other multiple access communications systemthat supports voice and data communication between users over aterrestrial link.

The use of CDMA techniques in a multiple access communications system isdisclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLEACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,”and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATINGWAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”. Another specific CDMAsystem is disclosed in U.S. Pat. No. 6,574,211, entitled “METHOD ANDAPPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” issued Jun. 3, 2003.These patents are assigned to the assignee of the present invention andincorporated herein by reference.

CDMA systems are typically designed to conform to one or more standardssuch as the “TIA/EIA/IS-95-A Mobile Station-Base Station CompatibilityStandard for Dual-Mode Wideband Spread Spectrum Cellular System”(hereinafter referred to as the IS-95-A standard), the “TIA/EIA/IS-98Recommended Minimum Standard for Dual-Mode Wideband Spread SpectrumCellular Mobile Station” (hereinafter referred to as the IS98 standard),the standard offered by a consortium named “3rd Generation PartnershipProject” (3GPP) and embodied in a set of documents including DocumentNos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214(hereinafter referred to as the W-CDMA standard), and the “TR-45.5Physical Layer Standard for cdma2000 Spread Spectrum Systems”(hereinafter referred to as the CDMA-2000 standard). New CDMA standardsare continually proposed and adopted for use. These CDMA standards areincorporated herein by reference.

FIG. 2 is a block diagram of modulator 116, which can be used to processa downlink data transmission in a Space-Time Spreading transmitdiversity mode in accordance with the CDMA-2000 standard (hereinafterreferred to as the STS mode). In the STS mode of the CDMA-2000 standard,the data symbols Y to be transmitted are provided to a demultiplexer(DEMUX) 208 and demultiplexed into two complex symbol streams, Y_(even)and Y_(odd), which are then provided to modulators 210 a and 210 b. Theeven complex symbol stream Y_(even) comprises the even inphase symbolstream Y_(I1) and the even quadrature symbol stream Y_(Q1). Similarly,the odd complex symbol stream Y_(odd) comprises the odd inphase symbolstream Y_(I2) and the odd quadrature symbol stream Y_(Q2). The evensymbol streams comprise “even” indexed data symbols and the odd symbolstreams comprise “odd” indexed data symbols. Each modulator 210 performschannelization (i.e., covering) and spreading of the even and odd symbolstreams and provides a complex output symbol stream S for a respectiveantenna.

In the non-transmit diversity (non-TD) mode of the CDMA-2000 standard,complex data symbols are transmitted serially, with each data symbolhaving a signaling period of T. In the STS mode, two complex datasymbols are transmitted in parallel over two antennas, with each datasymbol having a signaling period of 2T. As defined by the CDMA-2000standard, within each modulator 210, one of the complex symbol streams(even or odd) is covered with a Walsh symbol W_(STS) having a length of2T, and the other complex symbol stream (odd or even) is covered with acomplementary Walsh symbol W _(STS) having a length of 2T.

Within modulator 210 a, the even and odd complex symbol streams,Y_(even) and Y_(odd), are provided to symbol repeaters 212 a and 212 b,respectively. In the STS mode, each symbol repeater 212 repeats eachreceived data symbol once to double the signaling period from T to 2T.The symbol streams from symbol repeaters 212 a and 212 b are thenprovided to cover elements 214 a and 214 b, respectively, which coverthe data symbols with a channelization code associated with the physicalchannel used for the data transmission. In the STS mode, thechannelization code for cover element 214 a is the Walsh symbol W_(STS)having a length of 2T, and the channelization code for cover element 214b is the complementary Walsh symbol W _(STS) having the same length of2T. Each cover element 214 covers (e.g., multiplies) each received datasymbol with the Walsh symbol W_(STS) or W _(STS) in a manner known inthe art.

In the STS mode, the complex symbols from cover element 214 b areprovided to a complex conjugator 216 a that conjugates each receivedsymbol. The conjugated symbols from complex conjugator 216 a are thenprovided to a summer 218 a and subtracted from the symbols from coverelement 214 a to provide complex covered symbols. Each complex coveredsymbol thus includes a pair of data symbols that have been covered withthe Walsh symbols W_(STS) and W _(STS). The signal processing in the STSmode provides diversity in the transmitted signals, which can result inimproved performance.

In the embodiment shown in FIG. 2, the complex covered symbols fromsummer 218 a are provided to a phase rotator 222 a. In an embodiment,phase rotator 222 a provides a phase rotation of the received complexsymbols (e.g., in 90° increments) when enabled by a control signalROTATE. For example, if the received complex symbols are expressed asI_(C)+jQ_(C), phase rotator 222 a can provide 90° phase rotation of thecomplex symbols, which can then be expressed as −Q_(C)+jI_(c). The phaserotation allows modulator 210 a to account (i.e., compensate) for phaseshifts in the modulated signal due to switching or adjustments in thesubsequent signal conditioning circuitry within transmitter 118.

A complex multiplier 224 a then receives the phase rotated complexsymbol stream from phase rotator 222 a and a complex spreading sequencePN, spreads the complex symbol stream with the complex spreadingsequence, and provides a complex output symbol stream S₁. The complexspreading sequence PN is generated in a manner defined by the particularCDMA system or standard being implemented. For the CDMA-2000 system, thecomplex spreading sequence PN is generated by multiplying the short PNsequences, IPN and QPN, assigned to the transmitting base station withthe long PN sequence assigned to the receiving user terminal for whichthe data transmission is destined.

FIG. 3 is a diagram of a complex multiplier 300 that can be used toimplement each complex multiplier 224 in FIG. 2. Complex multiplier 300performs a complex multiply of the complex data symbols, D_(I)+jD_(Q),with the complex spreading sequence, PN_(I)+jPN_(Q), to provide complexspread output symbols, S_(I)+jS_(Q).

Within complex multiplier 300, the inphase data symbols D_(I) areprovided to multipliers 312 a and 312 b, and the quadrature data symbolsD_(Q) are provided to multipliers 312 c and 312 d. Each of themultipliers 312 a and 312 d also receives the inphase spreading sequencePN_(I), and each of multipliers 312 b and 312 c also receives thequadrature spreading sequence PN_(Q). Each multiplier 312 multiplies thereceived data symbols with the received spreading sequence and providesrespective spread symbols. A summer 314 a receives and subtracts theoutput from multiplier 312 c from the output from multiplier 312 a toprovide the inphase output symbols S_(I). A summer 314 b receives andcombines the outputs from multipliers 312 b and 312 d to provide thequadrature output symbols S_(Q).

Referring back to FIG. 2, modulator 210 b is configured similar tomodulator 210 a, with three differences. First, in modulator 210 b, thecomplementary Walsh symbol W _(STS) is used to cover the even complexsymbol stream Y_(even), and the Walsh symbol W_(STS) is used to coverthe odd complex symbol stream Y_(odd). Second, complex conjugator 216 bcouples to the output of cover element 214 c (i.e., the processing pathfor the even complex symbol stream Y_(even)). And third, the signs forthe inputs of summer 218 b are different than the signs for the inputsof summer 216 a in modulator 210 a.

The processing performed by modulator 116 can be described as follows.Initially, the even and odd complex symbol streams can be expressed as:Y _(even) =Y _(I1) +j Y _(Q1), and  Eq (1)Y _(odd) =Y _(I2) +j Y _(Q2).  Eq (2)As defined by the CDMA-2000 standard, the Walsh symbols W_(STS) and W_(STS) are used to cover the even and odd complex symbol streams. Eachof these Walsh symbols has a length of 2T and can be generated from aWalsh symbol W of length T as follows:W _(STS) =WW, and  Eq (3)W _(STS) =W W,where W=−W.

If only the data symbols and the covering are considered (i.e., ignoringthe PN spreading, phase rotation, transmit gain, pulse shaping, andother signal processing), the complex output symbol stream for antenna 1can be expressed as:S ₁ =Y _(even) WW−Y* _(odd) W W,  Eq (4)where the asterisk (*) denotes a complex conjugate operation. Similarly,the complex output symbol stream for antenna 2 can be expressed as:S ₂ =Y* _(even) W W+Y _(odd) WW.  Eq (5)

The complex output symbol streams, S₁ and S₂, are subsequently providedto two respective processing paths in transmitter 118. Each processingpath filters the inphase and quadrature symbol streams, S₁ and S_(Q), ofthe complex symbol stream S, modulates the filtered S₁ stream with aninphase carrier signal cos (ω_(c)t), modulates the filtered S_(Q) streamwith a quadrature carrier signal sin (ω_(c)t), sums the two modulatedcomponents, and further conditions the resultant signal to generate amodulated signal. In the STS mode, two modulated signals are generatedbased on two complex symbol streams S₁ and S₂, and are transmitted fromtwo antennas.

Typically, distinct (i.e., orthogonal) pilots are sent on respectivetransmit antennas. For example, for the CDMA-2000 system, an unmodulatedpilot (using Walsh code 0, 64) is sent on the common antenna and amodulated diversity pilot (using Walsh code 16, 128) is sent on thediversity antenna. The pilots are selected to be orthogonal so that theamplitude and phase of one or both signals transmitted from therespective antennas can be recovered.

The downlink signal processing for the CDMA-2000 standard is describedin further detail in the CDMA-2000 standard, and by M. Buehrer et al. ina paper entitled “Proposed Text for Space Time Spreading (STS) v0.3,”dated 1999, and incorporated herein by reference. This paper was adoptedinto the CDMA-2000 standard by the 3GPP2 standard body.

FIG. 4 is a block diagram of a conventional demodulator architecture 400capable of demodulating a downlink data transmission that has beenprocessed in the STS mode of the CDMA-2000 standard. In the STS mode,the received signal includes two modulated signals that have beentransmitted from two transmit antennas. The signal from each transmitantenna typically experiences different path conditions, due to thespatial separation of the transmit antennas, and arrives at the receiverunit distorted by the particular path conditions. At the receiver unit,two or more correlators (i.e., fingers) are used to receive anddemodulate the two transmitted signals. The demodulated symbols from thecorrelators are then combined to recover the transmitted symbols.

Initially, the received signal is conditioned (e.g., amplified,filtered, downconverted, quadrature demodulated, and so on) anddigitized to provide a complex sample stream comprised of inphasesamples I_(IN) and quadrature samples Q_(IN). The complex sample streamis provided to each correlator assigned to process the received signal.Each correlator receives, tracks, and processes a respective instance(i.e., a particular multipath) of the signal from one of the transmitantennas.

As shown in FIG. 4, a correlator 410 a is assigned to receive andprocess the signal from the first transmit antenna, and a correlator 410b is assigned to receive and process the signal from the second transmitantenna. Within correlator 410 a, the complex received samples (i.e.,I_(IN)+jQ_(IN)) are provided to a complex multiplier 412 a that alsoreceives a complex despreading sequence PN₁ (i.e., PN₁=PN_(I1)+jPN_(Q1))having a particular time offset assigned to correlator 410 a andmatching the time delay of the signal instance being processed. Complexmultiplier 412 a despreads the complex samples with the PN₁ sequence andprovides the complex despread samples (i.e., I_(D1)+jQ_(D1)) to adecover element 414 a. Decover element 414 a decovers the complexreceived samples with the Walsh symbol W_(STS) and provides complexdecovered symbols to each of the complex multipliers 420 a and 420 b.The decovering is achieved by multiplying the inphase (and quadrature)samples with the Walsh symbol W_(STS) and accumulating the results overthe length (2T) of the Walsh symbol W_(STS) to provide inphase (andquadrature) decovered symbols.

Complex multiplier 420 a then demodulates the complex decovered symbolswith a conjugated complex pilot ĥ*₁ (estimated from a pilot transmittedfrom a first transmit antenna) recovered by correlator 410 a. Similarly,complex multiplier 420 b demodulates the complex decovered symbols witha conjugated complex pilot ĥ*₂ (estimated from a pilot transmitted froma second transmit antenna) recovered by correlator 410 b. The outputfrom complex multiplier 420 a comprises the even complex symbol streamC_(even) ¹ that is provided to an accumulator 442 a within a combiner440. Similarly, the output from complex multiplier 420 b comprises theodd complex symbol stream C_(odd) ¹ that is provided to an accumulator442 b within combiner 440.

Within correlator 410 b, the complex received samples (i.e.,I_(IN)+jQ_(IN)) are despread by a complex multiplier 412 b with acomplex despreading sequence PN₂ (i.e., PN₂=PN_(I2)+jPN_(Q2)) having aparticular time offset assigned to correlator 410 b. The complexdespread samples (i.e., I_(D2)+jQ_(D2)) are decovered by decover element414 b with the complementary Walsh symbol W _(STS) and conjugated by acomplex conjugator 416. The conjugated symbols are then demodulated withthe complex pilot ĥ₂ by a complex multiplier 420 c, and furtherdemodulated with the negative complex pilot −ĥ₁ by a complex multiplier420 d. The output from complex multiplier 420 c comprises the evencomplex symbol stream C_(even) ² that is provided to accumulator 442 a,and the output from complex multiplier 420 d comprises the odd complexsymbol stream C_(odd) ² that is provided to accumulator 442 b.

Accumulator 442 a combines the even complex symbol streams, and C_(even)¹ and C_(even) ², from correlators 410 a and 410 b and provides the evenoutput symbol stream C_(even) (i.e., C_(even)=C_(I1)+jC_(Q1)).Similarly, accumulator 442 b combines the odd complex symbol streams,C_(odd) ¹ and C_(odd) ², from correlators 410 a and 410 b and providesthe odd output symbol stream C_(odd) (C_(odd)=C_(I2)+jC_(Q2)). Thesymbol streams C_(I1), C_(Q1), C_(I2), and C_(Q2) are estimates of thesymbol streams Y_(I1), Y_(Q1), Y_(I2), and Y_(Q2), respectively,generated within modulator 116 in FIG. 2 and expressed in equations (1)and (2).

Demodulator architecture 400 is described in further detail by A.Kogiantis et al. in a paper entitled “Downlink Improvement throughSpace-Time Spreading,” dated Aug. 5, 1999, and incorporated herein byreference. This paper was submitted to the 3GPP2 standard body foradoption into the CDMA-2000 standard.

Demodulator architecture 400 shown in FIG. 4 has several majordisadvantages. First, sharing of information between correlators isrequired to perform the pilot demodulation. Each correlator 410 performstwo complex multiplications to achieve the pilot demodulation. The firstcomplex multiplication is performed between the decovered symbols andthe complex pilot estimated by that correlator. The second complexmultiplication is performed between the decovered symbols and thecomplex pilot estimated by the other correlator. Demodulatorarchitecture 400 can be modified to share decovered symbols instead ofpilot estimates. However, in both cases, the need to share informationbetween correlators is highly undesirable in many circuit designs.Additional circuitry would likely be required to coordinate the sharingof information, which would lead to increased complexity and costs.

Second, if more than one multipath of any of the transmitted signals isprocessed, it is necessary to pair up correlators with the same pathdelay to perform the pilot demodulation. This requirement imposesconstraints on the use of the correlators and requires coordinationbetween the correlators.

Consequently, as a result of these disadvantages, system performance maybe compromised by the use of demodulator architecture 400.

FIG. 5 is a block diagram of a specific embodiment of a demodulatorarchitecture 500 of the invention, which is capable of demodulating adownlink data transmission that has been processed in the STS mode ofthe CDMA-2000 standard. Initially, the received signal is conditionedand digitized to provide a complex sample stream that is provided toeach of correlators 510 a and 510 b. Each correlator 510 receives,tracks, and demodulates a signal transmitted from one of the transmitantennas.

Within correlator 510 a, the complex received samples (i.e.,I_(IN)+jQ_(IN)) are despread by a complex multiplier 512 a with acomplex despreading sequence PN₁ having a particular time offsetassigned to correlator 510 a. The complex despread samples (i.e.,I_(D1)+jQ_(D1)) are then decovered by decover element 514 a with a Walshsymbol W having a length of T to provide decovered “half-symbols”. Thedecovering is achieved by multiplying the inphase (and quadrature)samples by the Walsh symbol W and accumulating the resultant samplesover the length (T) of the Walsh symbol W.

Referring back to FIG. 2, in the STS mode, each data symbol is coveredby the Walsh symbol W_(STS) or W _(STS) having a length of 2T, whichcorresponds to one STS symbol period. Also, referring to equation (3),the Walsh symbols W_(STS) and W _(STS) are generated by combining theWalsh symbol W and the complementary Walsh symbol W. The Walsh symbols Wand W each has a length of T, which is half the length of the Walshsymbols W_(STS) and W _(STS). Each decovered half-symbol from decoverelement 514 thus corresponds to only half of the STS symbol period.

The complex decovered half-symbols from decover element 514 a areprovided to a switch 520 a. Switch 520 a provides the decoveredhalf-symbols corresponding to the first half of the STS symbol period(switch 520 a in position A) to a delay element 522 a and the decoveredhalf-symbols corresponding to the second half of the STS symbol period(switch 520 a in position B) to summers 524 a and 524 b. Switch 520 acan be implemented with a demultiplexer, registers, latches, or someother element. Delay element 522 a delays the received half-symbols andprovides the delayed half-symbols to summers 524 a and 524 b. The delayis selected such that the decovered half-symbols for each STS symbolperiod are aligned in time at the inputs of each of summers 524 a and524 b.

For each STS symbol period of 2T (i.e., the length of the Walsh symbolsW_(STS) and W _(STS)) and after the decovered half-symbol correspondingto the second half of the STS symbol period has been received, summer524 a sums the two received half-symbols and provides the decoveredsymbol to a complex multiplier 528 a. Similarly, for each STS symbolperiod, summer 524 b subtracts the half-symbol received from switch 520a from the half-symbol received from delay element 522 a and providesthe decovered symbol to a complex conjugator 526 a. Complex conjugator526 a conjugates the received symbols and provides the conjugatedsymbols to a complex multiplier 528 b.

Complex multiplier 528 a demodulates the complex decovered symbols fromsummer 524 a with a conjugated complex pilot ĥ*₁ recovered by correlator510 a. Similarly, complex multiplier 528 b demodulates the complexdecovered symbols from complex conjugator 526 a with the negated complexpilot −ĥ₁. The output from complex multiplier 528 a comprises the evencomplex symbol stream C_(even) ¹ that is provided to an accumulator 542a within a combiner 540, and the output from complex multiplier 528 bcomprises the odd complex symbol stream C_(odd) ¹ that is provided to anaccumulator 542 b within combiner 540.

Correlator 510 b performs similar processing as correlator 510 a. Withincorrelator 510 b, the complex received samples (i.e., I_(IN)+jQ_(IN))are despread by a complex multiplier 512 b with a complex despreadingsequence PN₂ having a particular time offset assigned to correlator 510b. The complex despread samples are then decovered by decover element514 b with the Walsh symbol W to provide decovered half-symbols.

The complex decovered half-symbols from decover element 514 b areprovided to a switch 520 b, which provides decovered half-symbolscorresponding to the first half of the STS symbol period (switch 520 bin position A) to a delay element 522 b and decovered half-symbolscorresponding to the second half of the STS symbol period (switch 520 bin position B) to summers 524 c and 524 d. Delay element 522 b delaysthe received half-symbols and provides the delayed half-symbols tosummers 524 c and 524 d. Again, the delay is selected such that thedecovered half-symbols for each STS symbol period are time-aligned atthe inputs of each of summers 524 c and 524 d. For each STS symbolperiod, summer 524 c subtracts the half-symbol received from switch 520b from the half-symbol received from delay element 522 b and providesthe decovered symbol to a complex conjugator 526 b, which conjugates thereceived symbol and provides the conjugated symbol to a complexmultiplier 528 c. For each STS symbol period, summer 524 d sums the tworeceived half-symbols and provides the decovered symbol to a complexmultiplier 528 d.

Complex multiplier 528 c demodulates the complex decovered symbols fromcomplex conjugator 526 b with a complex pilot ĥ₂ recovered by correlator510 b. Similarly, complex multiplier 528 d demodulates the complexdecovered symbols from summer 524 d with the conjugated complex pilotĥ*₂. The output from complex multiplier 528 c comprises the even complexsymbol stream C_(even) ² that is provided to accumulator 542 a, and theoutput from complex multiplier 528 d comprises the odd complex symbolstream C_(odd) ² that is provided to accumulator 542 b.

Accumulator 542 a combines the even complex symbol streams, C_(even) ¹and C_(even) ², from correlators 510 a and 510 b and provides the evenoutput symbol stream C_(even) (i.e., C_(even)+C_(I1)+jC_(QI)).Similarly, accumulator 542 b combines the odd complex symbol streams,C_(odd) ¹ and C_(odd) ², from correlators 510 a and 510 b and providesthe odd output symbol stream C_(odd) (i.e., C_(odd)=C_(I2)+jC_(Q2)). Thesymbol streams C_(I1), C_(Q1), C_(I2), and C_(Q2) are estimates of thesymbol streams Y_(I1), Y_(Q1), Y_(I2), and Y_(Q2), respectively,generated within modulator 116 in FIG. 2.

The processing performed by demodulator architecture 500 can be analyzedby first characterizing the transmitted symbol streams. The transmittedsymbol streams, S₁ and S₂, in the STS mode are expressed above inequations (4) and (5). The Walsh symbols W_(STS) and W _(STS) of length2T can each be decomposed into a combination of Walsh symbols W and W,each of length T. The transmitted symbols can be decomposed into acombination of half-symbols transmitted over the first time interval T₁of the STS symbol period and half-symbols transmitted over the secondtime interval T₂ of the STS symbol period.

The transmitted symbols for the first antenna in equation (4) can beexpressed as:S ₁ =S ₁ ^(T1) , S ₁ ^(T2) ,S ₁ ^(T1) =Y _(even) W−Y* _(odd) W, and  Eq(6)S ₁ ^(T2) =Y _(even) W+Y* _(odd) W.

Similarly, the transmitted symbols for the second antenna in equation(5) can be expressed as:S ₂ =S ₂ ^(T1) , S ₂ ^(T2) ,S ₂ ^(T1) =Y* _(even) W+Y _(odd) W, and  Eq(7)S ₂ ^(T2) =−Y* _(even) W+Y _(odd) W.

The signals from the first and second transmit antennas are receivedwith random amplitudes and phases given by the complex values h₁ and h₂,respectively. The values h₁ and h₂ characterize the path loss andmultipath fading experienced by the transmitted signals. If the noise isignored, the composite received signal can be expressed as:R=S ₁ h ₁ , S ₂ h ₂ =R ^(T1) , R ^(T2) ,R ^(T1) =S ₁ ^(T1) h ₁ +S ₂ ^(T1) h ₂, and  Eq(8)R ^(T2) =S ₁ ^(T2) h ₁ +S ₂ ^(T2) h ₂,where R^(T1) and R^(T2) represent the received symbol waveforms for thefirst and second time intervals, T₁ and T₂, respectively, of the STSsymbol period. The even complex symbol streams C_(even) ¹ and C_(even) ²from correlators 510 a and 510 b, respectively, can be computed as:

$\begin{matrix}\begin{matrix}{C_{even}^{1} = {\left( {\left\langle {R^{T1},W} \right\rangle + \left\langle {R^{T2},W} \right\rangle} \right){\hat{h}}_{1}^{*}}} \\{= {\left( {\left\langle {\left( {{S_{1}^{T1}h_{1}} + {S_{2}^{T1}h_{2}}} \right),W} \right\rangle + \left\langle {\left( {{S_{1}^{T2}h_{1}} + {S_{2}^{T2}h_{2}}} \right),W} \right\rangle} \right){\hat{h}}_{1}^{*}}} \\{= {N\left( {{\left( {\left( {Y_{even} - Y_{odd}^{*}} \right) + \left( {Y_{even} + Y_{odd}^{*}} \right)} \right)h_{1}} +} \right.}} \\{\left. {\left( {\left( {Y_{even}^{*} + Y_{odd}} \right) + \left( {{- Y_{even}^{*}} + Y_{odd}} \right)} \right)h_{2}} \right){\hat{h}}_{1}^{*}} \\{= {2{N\left( {{Y_{even}h_{1}{\hat{h}}_{1}^{*}} + {Y_{odd}h_{2}{\hat{h}}_{1}^{*}}} \right)}}}\end{matrix} & {{Eq}\mspace{14mu}(9)} \\\begin{matrix}{C_{even}^{2} = {\left( {\left\langle {R^{T1},W} \right\rangle - \left\langle {R^{T2},W} \right\rangle} \right)^{*}{\hat{h}}_{2}}} \\{= {\left( {\left\langle {\left( {{S_{1}^{T1}h_{1}} + {S_{2}^{T1}h_{2}}} \right),W} \right\rangle - \left\langle {\left( {{S_{1}^{T2}h_{1}} + {S_{2}^{T2}h_{2}}} \right),W} \right\rangle} \right)^{*}{\hat{h}}_{2}}} \\{= {N\left( {{\left( {\left( {Y_{even} - Y_{odd}^{*}} \right) - \left( {Y_{even} + Y_{odd}^{*}} \right)} \right)^{*}h_{1}^{*}} +} \right.}} \\{\left. {\left( {\left( {Y_{even}^{*} + Y_{odd}} \right) - \left( {{- Y_{even}^{*}} + Y_{odd}} \right)} \right)^{*}h_{2}^{*}} \right){\hat{h}}_{2}^{*}} \\{= {2{N\left( {{{- Y_{odd}}h_{1}^{*}{\hat{h}}_{2}} + {Y_{even}h_{2}^{*}{\hat{h}}_{2}}} \right)}}}\end{matrix} & {{Eq}\mspace{14mu}(10)}\end{matrix}$where <R^(T1), W> denotes the decovering of the symbol waveform R^(T1)by the first correlator with the Walsh symbol W, 2N represents thelength of the Walsh symbols W_(STS) and W _(STS) (in chips), and(AB)*=A* B*. Similarly, the odd complex symbol streams C_(odd) ¹ andC_(odd) ² from correlators 510 a and 510 b, respectively, can becomputed as:

$\begin{matrix}\begin{matrix}{C_{odd}^{1} = {{- \left( {\left\langle {R^{T1},W} \right\rangle - \left\langle {R^{T2},W} \right\rangle} \right)^{*}}{\hat{h}}_{1}}} \\{= {{- \left( {\left\langle {\left( {{S_{1}^{T1}h_{1}} + {S_{2}^{T1}h_{2}}} \right),W} \right\rangle - \left\langle {\left( {{S_{1}^{T2}h_{1}} + {S_{2}^{T2}h_{2}}} \right),W} \right\rangle} \right)^{*}}{\hat{h}}_{1}}} \\{= {- {N\left( {{\left( {\left( {Y_{even} - Y_{odd}^{*}} \right) - \left( {Y_{even} + Y_{odd}^{*}} \right)} \right)^{*}h_{1}^{*}} +} \right.}}} \\{\left. {\left( {\left( {Y_{even}^{*} + Y_{odd}} \right) - \left( {{- Y_{even}^{*}} + Y_{odd}} \right)} \right)^{*}h_{2}^{*}} \right){\hat{h}}_{1}} \\{= {2{N\left( {{Y_{odd}h_{1}^{*}{\hat{h}}_{1}} - {Y_{even}h_{2}^{*}{\hat{h}}_{1}}} \right)}}}\end{matrix} & {{Eq}\mspace{14mu}(11)} \\\begin{matrix}{C_{odd}^{2} = {\left( {\left\langle {R^{T1},W} \right\rangle + \left\langle {R^{T2},W} \right\rangle} \right){\hat{h}}_{2}^{*}}} \\{= {\left( {\left\langle {\left( {{S_{1}^{T1}h_{1}} + {S_{2}^{T1}h_{2}}} \right),W} \right\rangle + \left\langle {\left( {{S_{1}^{T2}h_{1}} + {S_{2}^{T2}h_{2}}} \right),W} \right\rangle} \right){\hat{h}}_{2}^{*}}} \\{= {N\left( {{\left( {\left( {Y_{even} - Y_{odd}^{*}} \right) + \left( {Y_{even} + Y_{odd}^{*}} \right)} \right)h_{1}} +} \right.}} \\{\left. {\left( {\left( {Y_{even}^{*} + Y_{odd}} \right) + \left( {{- Y_{even}^{*}} + Y_{odd}} \right)} \right)h_{2}} \right){\hat{h}}_{2}^{*}} \\{= {2{N\left( {{Y_{even}h_{1}{\hat{h}}_{2}^{*}} + {Y_{odd}h_{2}{\hat{h}}_{2}^{*}}} \right)}}}\end{matrix} & {{Eq}\mspace{14mu}(12)}\end{matrix}$

The even complex symbol stream C_(even) from combiner 542 a and the oddcomplex symbol stream C_(odd) from combiner 542 b can be expressed as:

$\begin{matrix}\begin{matrix}{{C_{even} = {C_{even}^{1} + C_{even}^{2}}},} \\{= {{2{N\left( {{Y_{even}h_{1}{\hat{h}}_{1}^{*}} + {Y_{odd}h_{2}{\hat{h}}_{1}^{*}}} \right)}} +}} \\{{2{N\left( {{{- Y_{odd}}h_{1}^{*}{\hat{h}}_{2}} + {Y_{even}h_{2}^{*}{\hat{h}}_{2}}} \right)}},} \\{{= {2{N\left( {{Y_{even}\left( {{h_{1}{\hat{h}}_{1}^{*}} + {h_{2}^{*}{\hat{h}}_{2}}} \right)} + {Y_{odd}\left( {{h_{2}{\hat{h}}_{1}^{*}} - {h_{1}^{*}{\hat{h}}_{2}}} \right)}} \right)}}},}\end{matrix} & {{Eq}\mspace{14mu}(13)} \\\begin{matrix}{{C_{odd} = {C_{odd}^{1} + C_{odd}^{2}}},} \\{= {{2{N\left( {{Y_{odd}h_{1}^{*}{\hat{h}}_{1}} - {Y_{even}h_{2}^{*}{\hat{h}}_{1}}} \right)}} +}} \\{{2{N\left( {{Y_{even}h_{1}{\hat{h}}_{2}^{*}} + {Y_{odd}h_{2}{\hat{h}}_{2}^{*}}} \right)}},} \\{= {2{{N\left( {{Y_{odd}\left( {{h_{1}^{*}{\hat{h}}_{1}} + {h_{2}{\hat{h}}_{2}^{*}}} \right)} + {Y_{even}\left( {{h_{1}{\hat{h}}_{2}^{*}} - {h_{2}^{*}{\hat{h}}_{1}}} \right)}} \right)}.}}}\end{matrix} & {{Eq}\mspace{14mu}(14)}\end{matrix}$In each of equations (13) and (14), the first term is the desired signalcomponent and the second term is the undesired component due tocross-talk. If the pilot estimates are accurate (i.e., ĥ₁=h₁ and ĥ₂=h₂),then equations (13) and (14) simplify as followsC _(even)=2N Y _(even) (|h ₁|² +|h ₂|²),  Eq (15)C _(odd)=2N Y _(odd) (|h₁|² +|h ₂|²).  Eq (16)

Demodulator architecture 500 can recover the transmitted symbols if onetransmit antenna should fail to operate or if the signal transmittedfrom one of the antennas experiences a deep fade. As an example, if thesecond transmit antenna should fail, the received symbol stream can beexpressed as:R=S₁h₁,R^(T1)=S₁ ^(T1)h₁, and  Eq(17)R^(T2)=S₁ ^(T2)h₁.

At the receiver unit, one correlator can be used to receive and processthe transmitted signal. The even and odd complex symbol streams,C_(even) and C_(odd), from the assigned correlator can be expressed as:

$\begin{matrix}\begin{matrix}{{C_{even}^{1} = {\left( {\left\langle {R^{T1},W} \right\rangle + \left\langle {R^{T2},W} \right\rangle} \right){\hat{h}}_{1}^{*}}},} \\{{= {\left( {\left\langle {\left( {S_{1}^{T1}h_{1}} \right),W} \right\rangle + \left\langle {\left( {S_{1}^{T2}h_{1}} \right),W} \right\rangle} \right){\hat{h}}_{1}^{*}}},} \\{{= {{N\left( {\left( {\left( {Y_{even} - Y_{odd}^{*}} \right) + \left( {Y_{even} + Y_{odd}^{*}} \right)} \right)h_{1}} \right)}{\hat{h}}_{1}^{*}}},} \\{= {2{{N\left( {Y_{even}h_{1}{\hat{h}}_{1}^{*}} \right)}.}}}\end{matrix} & {{Eq}\mspace{14mu}(18)} \\\begin{matrix}{C_{odd}^{1} = {{- \left( {\left\langle {R^{T1},W} \right\rangle - \left\langle {R^{T2},W} \right\rangle} \right)^{*}}{\hat{h}}_{1}}} \\{= {{- \left( {\left\langle {\left( {S_{1}^{T1}h_{1}} \right),W} \right\rangle - \left\langle {\left( {S_{1}^{T2}h_{1}} \right),W} \right\rangle} \right)^{*}}{\hat{h}}_{1}}} \\{= {{- {N\left( {\left( {\left( {Y_{even} - Y_{odd}^{*}} \right) - \left( {Y_{even} + Y_{odd}^{*}} \right)} \right)^{*}h_{1}^{*}} \right)}}{\hat{h}}_{1}}} \\{= {2{N\left( {Y_{odd}h_{1}^{*}{\hat{h}}_{1}} \right)}}}\end{matrix} & {{Eq}\mspace{14mu}(19)}\end{matrix}$Again, if the pilot estimate is accurate (i.e., ĥ₁=h₁), then equations(18) and (19) simplify as follows:C _(even) ¹=2N Y _(even)(|h₁|²),C _(odd) ¹=2N Y _(odd) (|h₁|²).

Demodulator architecture 500 shown in FIG. 5 provides a number ofadvantages over demodulator architecture 400 shown in FIG. 4. Theseadvantages can result in a simplified design, reduced costs, improvedperformance, some other advantages, or a combination thereof. Some ofthese advantages are described below.

First, demodulator architecture 500 does not require the sharing ofpilot estimates and data symbols between correlators. Each correlatorreceives, processes, and demodulates the received sample stream with itsown pilot estimate. The autonomous design for the correlators eliminatesthe need to transfer information between correlators and simplifies thedesign of the receiver unit that uses demodulator architecture 500.

Second, demodulator architecture 500 does not require correlators to bepaired up. This allows for flexibility in assigning correlators to thestrongest signal instances, which can lead to improved performance.

Third, demodulator architecture 500 does not require synchronization ofthe pilots of paired correlators that have unequal path delays. Thisfeature results from the ability of each correlator to operateindependently based on the received samples and its own pilot estimate.In contrast, since the correlators are operated in pairs in demodulatorarchitecture 400, the pilots needs to be properly aligned in time toaccount for any delays between the signal instances being processed bythe pair of correlators.

Fourth, demodulator architecture 500 allows for reception of thetransmitted symbols if one of the transmit antennas should fail tooperate or is in a deep fade. In contrast, demodulator architecture 400can only recover half of the transmitted symbol should one transmitantenna fail. Demodulator architecture 500 can be used to provide a morerobust and reliable communication.

FIG. 6 is a block diagram of another specific embodiment of ademodulator architecture 600 of the invention, which is also capable ofdemodulating a downlink data transmission that has been processed in theSTS mode of the CDMA-2000 standard. The complex sample stream isprovided to correlators 610 a and 610 b, with each correlator 610operated to receive, track, and demodulate a signal transmitted from oneof the transmit antennas.

Within correlator 610 a, the complex received samples are despread by acomplex multiplier 612 a with a despreading sequence PN₁ and decoveredby a decover element 614 a with the Walsh symbol W to provide decoveredhalf-symbols. The decovered half-symbols are then demodulated with aconjugated complex pilot ĥ*₁ recovered by correlator 610 a to providedemodulated half-symbols, which are then provided to a switch 620 a. Inthe first half of the STS symbol period, switch 620 a is in position A,and the demodulated half-symbol is provided to a signal path 622 a andthe inverted demodulated half-symbol is provided to a signal path 622 b.In the second half of the STS symbol period, switch 620 a is in positionB, and the demodulated half-symbol is provided to signal paths 622 a and622 b. Switch 620 a can be implemented with a demultiplexer or someother element.

The demodulated half-symbols on signal path 622 a are provided to anaccumulator 642 a within a combiner 640. The demodulated half-symbols onsignal path 622 b are provided to a complex conjugator 626 a, whichconjugates the received half-symbols and provides the conjugatedhalf-symbols to an accumulator 642 b within combiner 640.

Correlator 610 b processes the complex received samples in similarmanner as correlator 610 a. Specifically, correlator 610 b despreads thecomplex received samples with a despreading sequence PN₂, decovers thedespread samples with the Walsh symbol W to provide decoveredhalf-symbols, and demodulates the decovered half-symbols with aconjugated complex pilot ĥ*₂ recovered by correlator 610 b to providedemodulated half-symbols. The demodulated half-symbols corresponding tothe first half of the STS symbol period are provided to accumulator 642b, and also conjugated and provided to accumulator 642 a. Similarly, thedemodulated half-symbols corresponding to the second half of the STSsymbol period are provided to accumulator 642 b, and also inverted andconjugated and provided to accumulator 642 a.

For each STS symbol period, accumulator 642 a combines the four receiveddemodulated half-symbols and provides an even output symbol, andaccumulator 642 b combines the four received demodulated half-symbols toprovide an odd output symbol.

Demodulator architecture 600 generates equivalent results as demodulatorarchitecture 500 in FIG. 5. However, by performing the pilotdemodulation after the decovering, only one complex multiplier isrequired. Complex multiplier 616 performs one complex multiply (e.g.,one dot product and one cross product) for each half of the STS symbolperiod (i.e., each period of T). In contrast, each of multipliers 528 indemodulator architecture 500 performs one complex multiply for each STSsymbol period of 2T.

Also, the summers (i.e., summers 524) used to combine the decoveredhalf-symbols for each STS symbol period are not needed in demodulatorarchitecture 600 since this function is performed by accumulators 642 aand 642 b. Each accumulator 642 performs twice the number ofread-accumulate-write operations for each STS symbol period asaccumulator 542 in demodulator architecture 500.

FIG. 7 is a block diagram of yet another specific embodiment of ademodulator architecture 700 of the invention, which is also capable ofdemodulating a downlink data transmission that has been processed in theSTS mode of the CDMA-2000 standard. The complex sample stream isprovided to correlators 710 a and 710 b, with each correlator 710operated to receive, track, and demodulate a signal transmitted from oneof the transmit antennas.

Within each correlator 710, the complex received samples are despread bya complex multiplier 712 with a despreading sequence PN having aparticular time offset assigned to that correlator, decovered by adecover element 714 with the Walsh symbol W to provide decoveredhalf-symbols, and demodulated by a complex multiplier 716 with aconjugated complex pilot ĥ* recovered by that correlator to providedemodulated half-symbols.

Within correlator 710 a, a switch 720 a provides demodulatedhalf-symbols corresponding to the first half of the STS symbol period toan accumulator 742 a within a combiner 740 and further providesdemodulated half-symbols corresponding to the second half of the STSsymbol period to an accumulator 742 b within combiner 740. Similarly,within correlator 710 b, a switch 720 b provides demodulatedhalf-symbols corresponding to the first half of the STS symbol period toan accumulator 742 c and demodulated half-symbols corresponding to thesecond half of the STS symbol period to an accumulator 742 d. Eachaccumulator 742 selectively combines the received half-symbols toprovide the output symbols.

In FIG. 7, complex multipliers 716 a and 716 b are each configured toperform two complex multiplies for each STS symbol period. The complexmultiply from correlator n for time interval Tx of the STS symbol periodcan be expressed as:

$\begin{matrix}\begin{matrix}{{C^{n} = {\left( {X_{I} + {j\; X_{Q}}} \right)\left( {P_{I} - {j\; P_{Q}}} \right)}},} \\{{= {\left( {{X_{I}P_{I}} + {X_{Q}P_{Q}}} \right) + {j\left( {{X_{Q}P_{I}} - {X_{I}P_{Q}}} \right)}}},} \\{{= {C_{dot}^{n,{Tx}} + {j\; C_{cross}^{n,{Tx}}}}},}\end{matrix} & {{Eq}\mspace{14mu}(20)}\end{matrix}$

where X_(I)+jX_(Q) is the complex decovered half-symbol to bedemodulated, P_(I)−jP_(Q) is the conjugated pilot estimate (e.g.,ĥ*=P_(I)−jP_(Q)), and

$\begin{matrix}C_{dot}^{n,{Tx}} & \begin{matrix}{and} & C_{cross}^{n,{Tx}}\end{matrix}\end{matrix}$are the dot and cross products, respectively, for the complex multiply.

As shown in equation (20), each complex multiply can be performed with adot product and a cross product. The four complex multiplies performedby multipliers 716 a and 716 b for each STS symbol period can beachieved with four dot products and four cross products, which yieldfour “dot” symbols and four “cross” symbols, respectively. The dot andcross symbols are also referred to as intermediate symbols. In anembodiment, the eight intermediate symbols for each STS symbol periodcan be stored to eight memory locations and later combined when thesymbols are retrieved from memory.

The symbol combination performed by accumulators 742 can be computed asfollows. In correlator 710 a, the dot and cross products generate theintermediate symbols,

$\begin{matrix}C_{dot}^{1,{T1}} & {and} & {C_{cross}^{1,{T1}},}\end{matrix}$respectively, in the first half of the STS symbol period and theintermediate symbols

$\begin{matrix}C_{dot}^{1,{T2}} & {and} & {C_{cross}^{1,{T2}},}\end{matrix}$respectively, in the second half of the STS symbol period. Similarly, incorrelator 710 b, the dot and cross products generate the intermediatesymbols

C_(dot)^(2, T1)  and  C_(cross)^(2, T1),respectively, in the first half of the STS symbol period and theintermediate symbols

C_(dot)^(2, T 2)  and  C_(cross)^(2, T 2 ),respectively, in the second half of the STS symbol period. The evencomplex output symbols C_(even) can be expressed as:

$\begin{matrix}\begin{matrix}{{C_{even} = {C_{even}^{I} + {j\; C_{even}^{Q}}}},} & \; \\{{C_{even}^{I} = {C_{dot}^{1,{T1}} + C_{dot}^{1,{T2}} + C_{dot}^{2,{T1}} - C_{dot}^{2,{T2}}}},} & \; \\{C_{even}^{Q} = {C_{cross}^{1,{T1}} + C_{cross}^{1,{T2}} - C_{cross}^{2,{T1}} + {C_{cross}^{2,{T2}}.}}} & {and}\end{matrix} & {{Eq}\mspace{14mu}(21)}\end{matrix}$

Similarly, the odd complex output symbols C_(odd) can be expressed as:

$\begin{matrix}\begin{matrix}{{C_{odd} = {C_{odd}^{I} + {j\; C_{odd}^{Q}}}},} & \; \\{{C_{odd}^{I} = {{- C_{dot}^{1,{T1}}} + C_{dot}^{1,{T2}} + C_{dot}^{2,{T1}} + C_{dot}^{2,{T2}}}},} & \; \\{C_{odd}^{Q} = {C_{cross}^{1,{T1}} - C_{cross}^{1,{T2}} + C_{cross}^{2,{T1}} + {C_{cross}^{2,{T2}}.}}} & {and}\end{matrix} & {{Eq}\mspace{14mu}(22)}\end{matrix}$

To further simplify the computations, equations (21) and (22) may beexpressed as:

$\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}{{C_{even}^{I} = {\left( {C_{dot}^{1,{T1}} + C_{dot}^{1,{T2}} + C_{dot}^{2,{T1}} + C_{dot}^{2,{T2}}} \right) - {2C_{dot}^{2,{T2}}}}},} \\{{C_{even}^{Q} = {\left( {C_{cross}^{1,{T1}} + C_{cross}^{1,{T2}} + C_{cross}^{2,{T1}} + C_{cross}^{2,{T2}}} \right) - {2C_{cross}^{2,{T1}}}}},}\end{matrix} \\{{C_{odd}^{I} = {\left( {C_{dot}^{1,{T1}} + C_{dot}^{1,{T2}} + C_{dot}^{2,{T1}} + C_{dot}^{2,{T2}}} \right) - {2C_{dot}^{1,{T1}}}}},}\end{matrix} \\{C_{odd}^{Q} = {\left( {C_{cross}^{1,{T1}} + C_{cross}^{1,{T2}} + C_{cross}^{2,{T1}} + C_{cross}^{2,{T2}}} \right) - {2{C_{cross}^{1,{T2}}.}}}}\end{matrix} & {{Eq}\mspace{14mu}(23)}\end{matrix}$

In equation (23), the quantity within the parenthesis can be computedonce for the dot products and once for the cross products for each STSsymbol period. Two such combined symbols can be computed for each STSsymbol period. For each output symbol (e.g., C_(even) ¹), acorresponding intermediate symbol (e.g.,

C_(dot)^(2, T2)is scaled by a factor of two (e.g., shifted left by one bit) andsubtracted from a corresponding combined symbol (e.g.,

C_(dot)^(1, T1) + C_(dot)^(1, T2) + C_(dot)^(2, T1) + C_(dot)^(2, T2)

FIGS. 5, 6, and 7 show three specific embodiments of the presentinvention. Other embodiments can also be designed and are within thescope of the present invention. Generally, the demodulator architecturesof the present invention perform partial processing (e.g., despreading,decovering, pilot demodulation, or a combination thereof) on fractions(e.g., half, quarter, and so on) of the STS symbol period to generateprocessed “partial-symbols”. The processed partial-symbols are thenappropriately further processed and combined to generate the outputsymbols. By performing partial processing on each fraction of the STSsymbol period, numerous benefits described above are achieved.

The present invention has been described with designs in which thepartial processing is performed on half-symbols. However, partialprocessing on other fractions of the symbol period may also be performedand are within the scope of the present invention. For example, thepartial processing may be performed on quarter symbol period, eighthsymbol period, or some other fraction.

In the embodiments shown in FIGS. 5, 6, and 7, two correlators are usedto process the two signals transmitted from two antennas. Each of thesecorrelators can be operated to track the timing corresponding to thesignal instances being processed.

The signals from the two transmit antennas may also be processed basedon the same timing (e.g., the timing of one of the signal instancesbeing processed, or the average timing of the two signal instances, orothers). In this implementation, the same symbols are used for bothtransmitted signals, and the processing can be performed by a single(modified) correlator. The modified correlator can be designed toperform despreading and decovering with a particular time offset, andtwo pilot demodulation. Common sampling, decimation, despreading, anddecovering are performed for both transmitted signals. The use of thesame timing may result in higher cancellation of cross-talk, which canprovide improved performance.

The demodulator architectures of the invention can be employed invarious receiver architectures such as, for example, a rake receiver.The design and operation of a rake receiver for a CDMA system isdescribed in further detail in U.S. Pat. No. 5,764,687, entitled “Mobiledemodulator architecture for a spread spectrum multiple accesscommunication system,” and U.S. Pat. No. 5,490,165, entitled“Demodulation element assignment in a system capable of receivingmultiple signals,” both assigned to the assignee of the presentinvention and incorporated herein by reference.

The rake receiver typically includes many correlators (i.e., fingers)that are assigned to process strong instances of the received signal.The demodulator architectures of the invention allow for easy combiningof symbols or half-symbols from multiple assigned correlators. Forexample, referring back to FIG. 4, the even complex symbols from eachassigned correlator are provided to accumulator 442 a and the oddcomplex symbols from each assigned correlator are provided toaccumulator 442 b. For each STS symbol period, each accumulator 442combines all received symbols and provides a complex output symbol.Generally, the rake receiver using the demodulator architectures of theinvention can be designed to include as many correlators as desired.Each accumulator is then designed to accumulate symbols from allassigned correlators.

The processing to recover the transmitted pilot is known in the art andnot described in detail herein. The pilot processing is dependent in theparticular CDMA system or standard being implemented. For example,different pilot processing is typically performed depending on whetherthe pilot is added to (i.e., superimposed over) the data or timedivision multiplexed with the data. An example of the pilot processingis described in the aforementioned U.S. Pat. Nos. 5,764,687 and5,490,165.

For clarity, the demodulator architectures, demodulators, and receiverunits of the invention have been described specifically for the STS modedefined by the CDMA2000 standard. The invention can also be used inother communications systems that employ the same, similar, or differenttransmit diversity modes. The demodulator architecture of the inventioncan be used to provide the basic functionality (e.g., decovering, pilotdemodulation, and so on). Modification of the basic functionality and/oradditional processing (e.g., combining, reordering of the symbols, andso on) may be implemented to achieve the desired results.

For example, the W-CDMA standard provides a space time block codingtransmit antenna diversity (STTD) mode in which symbols are transmittedredundantly over two antennas. In the STTD mode, data symbols areredundantly sent to two modulators, but the symbols provided to thesecond modulator are reordered, with respect to the symbols provided tothe first modulator, in accordance with a particular ordering scheme. Tosupport the STTD mode, demodulator architectures of the invention can bemodified to temporarily store the demodulated symbols from the assignedcorrelators, reorder the symbols in the inverse manner, and combine thesymbols to recover the transmitted symbols.

The demodulator architectures described above can be advantageously usedin a user terminal (e.g., a mobile unit, a telephone, and so on) of acommunications system, and may also be used at a base station. Thesignal processing for the downlink and uplink may be different and istypically dependent on the particular CDMA standard or system beingimplemented. Thus, the demodulator architectures are typically adoptedespecially for the particular application for which it is used.

Some or all of the elements described above for the demodulatorarchitectures of the invention (e.g., the complex multipliers, decoverelements, switches, delay elements, summers, combiner, and so on) can beimplemented within one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), controllers,micro-controllers, microprocessors, programmable logic devices (PLDs),other electronic units designed to perform the functions describedherein, or a combination thereof. Also, some or all of the elementsdescribed above can be implemented using software or firmware executedon a processor.

As an example, a demodulator can be designed in which the despreader anddecoverer elements for each correlator are implemented in hardware, andthe pilot demodulation and symbol accumulation for all correlators areperformed by a DSP in a time division multiplexed manner. As anotherexample, one correlator and combiner can be implemented and used toprocess samples corresponding to various signal instances in a timedivision multiplexed manner. Numerous other implementations can becontemplated and are within the scope of the present invention.

The foregoing description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of theinventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. An apparatus for a communication system, comprising: a decoverelement for decovering a plurality of received samples to providedecovered half-symbols, wherein the decover element is configured toperform decovering with a decovering channelization symbol having alength (T) that is half the length (2T) of a covering channelizationsymbol used to cover the received samples; and a first multiplier forreceiving the decovered half-symbols and pilot symbols to providedemodulated half-symbols.
 2. The apparatus of claim 1, wherein thereceived samples are despread received samples, further comprising: asecond multiplier for producing the despread received samples.
 3. Theapparatus of claim 1, further comprising: a combiner for combining thedemodulated half-symbols received from the first multiplier.
 4. Theapparatus of claim 3, wherein the combiner comprises: a firstaccumulator for accumulating the demodulated half-symbols correspondingto a first half of a symbol period; and a second accumulator foraccumulating the demodulated half-symbols corresponding to a second halfof the symbol period.
 5. The apparatus of claim 1, further comprising: aswitch for selectively outputting the demodulated half-symbolscorresponding to a first half of the symbol period and the demodulatedhalf-symbols corresponding to a second half of the symbol period.
 6. Acommunication system, comprising: a transmitter; and a receiver forprocessing a received signal transmitted from the transmitter, saidreceiver including: a decover element for decovering a plurality ofreceived samples to provide decovered half-symbols, wherein the decoverelement is configured to perform decovering with a decoveringchannelization symbol having a length (T) that is half the length (2T)of a covering channelization symbol used to cover the received samples;and a first multiplier for receiving the decovered half-symbols andpilot symbols to provide demodulated half-symbols.
 7. The communicationsystem of claim 6, wherein the received samples are despread receivedsamples, further comprising: a second multiplier for producing thedespread received samples.
 8. The communication system of claim 6,further comprising: a combiner for combining the demodulatedhalf-symbols received from the first multiplier.
 9. The communicationsystem of claim 8, wherein the combiner comprises: a first accumulatorfor accumulating the demodulated half-symbols corresponding to a firsthalf of a symbol period; and a second accumulator for accumulating thedemodulated half-symbols corresponding to a second half of the symbolperiod.
 10. The communication system of claim 6, further comprising: aswitch for selectively outputting the demodulated half-symbolscorresponding to a first half of the symbol period and the demodulatedhalf-symbols corresponding to a second half of the symbol period.
 11. Amethod for processing a received signal in a wireless communicationsystem, comprising: decovering a plurality of received samples toprovide decovered half-symbols, wherein the decovering is performed witha decovering channelization symbol having a length (T) that is half thelength (2T) of a covering channelization symbol used to cover thereceived samples; and receiving the decovered half-symbols and pilotsymbols to provide demodulated half-symbols by a multiplier.
 12. Themethod of claim 11, further comprising: despreading the receivedsamples.
 13. The method of claim 11, further comprising: combining thedemodulated half-symbols received from the multiplier.
 14. The method ofclaim 13, wherein said combining further comprises: accumulating thedemodulated half-symbols corresponding to a first half of a symbolperiod in a first accumulator; and accumulating the demodulatedhalf-symbols corresponding to a second half of the symbol period in asecond accumulator.
 15. The method of claim 11, further comprising:selectively outputting the demodulated half-symbols corresponding to afirst half of the symbol period and the demodulated half-symbolscorresponding to a second half of the symbol period.